1. Field of the Invention
The present invention relates to a Faraday rotator used for an optical communication system, an optical device, such as an optical isolator, using the Faraday rotator, and an optical communication system including the optical device.
2. Description of the Related Art
At present, in connection with electric communication having a low transmission capacity, the spread of optical communication has been accelerated. As described below, the reason is summarized such that the optical communication enables high speed and large capacity transmission, is advantageous in long distance transmission since the number of relays may be small, and is not influenced by electromagnetic noise.
Light is identical to an electric wave used for TV/radio broadcast or wireless communication in electromagnetic waves. However, the frequency of the electromagnetic wave used for the optical communication is about 200 THz and is about 20000 times as high as that of satellite broadcast (about 10 GHz). That the frequency is high means that the wavelength is short, and a large number of signals can be transmitted at high speed all the more. Incidentally, the wavelength (center wavelength) of the electromagnetic wave used for the optical communication is 1.31 μm and 1.55 μm.
As is well known, an optical fiber used for the optical communication has a double structure of glasses having different refractive indexes. Since light transmitted through the center core repeats reflection in the inside of the core, even if the optical fiber is bent, a signal is accurately transmitted. Further, since high purity quartz glass having high transparency is used for the optical fiber, attenuation in the optical communication is no more than about 0.2 dB per km. Accordingly, transmission for about 100 km is enabled without using an amplifier, and the number of relays can be reduced as compared with the electric communication.
Although EMI (Electromagnetic Interference) becomes a problem in the electric communication, the communication using the optical fiber is not influenced by noise by electromagnetic induction. Thus, very high quality information transmission can be made.
In the present optical communication system, an electric signal is converted into an optical signal by an LD (Laser Diode) of an optical transmitter, this optical signal is transmitted through an optical fiber, and then, it is converted into an electric signal by a PD (Photo Diode) of an optical receiver. As stated above, elements indispensable to the optical communication system are the LD, the PD, the optical fiber and the optical connector. Apart from a relatively low speed and near distance communication system, in a high speed and long distance communication system, in addition to the above elements, it becomes necessary to provide an optical transmission equipment such as an optical amplifier or an optical distributor, and an optical part (optical device) applied to the equipment, such as an optical isolator, an optical circulator, an optical coupler, an optical separator, an optical switch, an optical modulator, or an optical attenuator.
In high speed and long distance transmission or a multi-branching optical communication system, the optical isolator is especially important. In the present optical communication system, the optical isolator is used in the LD module of the optical transmitter and the relay. The optical isolator is an optical part having a function to transmit an electromagnetic wave only in one direction and to block an electromagnetic wave returned by reflection on the way. The optical isolator employs a Faraday effect as a kind of magneto-optical effect. The Faraday effect is a phenomenon in which a polarization plane of light having passed through a Faraday rotator formed of a material exhibiting the Faraday effect, that is, a rare earth iron garnet single crystal film or the like is rotated. The property that the polarization direction of light is rotated, such as the Faraday effect, is called optical activity. However, differently from normal optical activity, in the Faraday effect, even if the traveling direction of light is reversed, a state is not restored, and the polarization direction is further rotated. An element using the phenomenon that the polarization direction of light is rotated by the Faraday effect is called a Faraday rotator.
The function of the optical isolator will be described while an LD module is used as an example.
An LD is made the LD module in which it is integrated with an optical fiber and is incorporated in an optical transmitter. An optical isolator is disposed between the LD and the Optical fiber, and functions to check reflected return light to the LD by using the Faraday effect. The reflected return light is light that is returned after light emitted from the LD is slightly reflected by a part such as an optical connecter. The reflected return light causes noise to be generated in the LD. The optical isolator allowing light to pass only in one direction removes this noise and keeps communication quality.
In the case of the LD of the optical transmitter, since the oscillation direction (polarization direction) of light emitted from the LD is determined to be one direction, a polarization dependency type optical isolator with a simple structure is used. The basic structure of a conventional polarization dependency type optical isolator 10 is shown in FIG. 19. The optical isolator 10 includes a Faraday rotator 11 made of a garnet single crystal film, a cylindrical permanent magnet 12 surrounding the Faraday rotator 11 and magnetizing the Faraday rotator 11, and polarizers 13 and 14 disposed on both front and back surfaces of the Faraday rotator 11. The polarizers 13 and 14 are disposed so that their polarizing axes have a relative angle of 45°. Incidentally, in the optical isolator 10, a direction in which light travels is called a forward direction, and a direction in which light is reflected and returned is called a reverse direction.
The Faraday rotator affects the performance of the optical isolator. Accordingly, the property of a material constituting the Faraday rotator is important for obtaining the high performance optical isolator. Important points of selection of the material constituting the Faraday rotator are that a Faraday rotation angle at a use wavelength (in the case of an optical fiber, 1.31 μm, 1.55 μm) is large, and transparency is high. As the material satisfying such conditions, YIG (yttrium iron garnet, Y3Fe5O12) was used at the beginning, however, it was insufficient in mass production and miniaturization.
Thereafter, it has been found that when a rare earth site of garnet crystal is substituted with bismuth (Bi), the Faraday rotational capacity is remarkably improved, and after this, this Bi substitutional rare earth iron garnet single crystal has been used for the Faraday rotator.
Incidentally, in the conventional bismuth substitutional rare earth iron garnet single crystal, the Faraday rotation angle indicates a definite value in a magnetic field not lower than the saturation magnetic field. In the magnetic field lower than the saturation magnetic field, the Faraday rotation angle is in proportion to the magnitude of the magnetic field, and when the external magnetic field is removed, the Faraday effect disappears. Thus, as shown in FIG. 19, in the conventional optical isolator 10, the permanent magnet 12 for applying the magnetic field not lower than the saturation magnetic field to the Faraday rotator 11 is disposed.
Also with respect to the optical isolator 10, similarly to the other equipments and parts, there is a demand for miniaturization and cost reduction. However, it can be said that the existence of this permanent magnet 12 prevents the miniaturization and cost reduction of the optical isolator 10.
In the conventional bismuth substitutional rare earth iron garnet single crystal, when the external magnetic field is removed, the Faraday effect disappears, and therefore, it can be said that the single crystal is a soft magnetic material. Thus, the disposition of the permanent magnet 12 is indispensable. However, if hard magnetism, that is, a property (latching) capable of keeping the Faraday rotation angle even if the external magnetic field is removed can be given to the bismuth substitutional rare earth iron garnet single crystal, the disposition of the permanent magnet 12 can be omitted. The omission of the permanent magnet 12 produces the miniaturization and cost reduction of various equipments and parts using the optical isolator or the Faraday effect. Thus, the bismuth substitutional rare earth iron garnet single crystal has been developed.
For example, JP-A-6-222311 discloses a bismuth substitutional rare earth iron garnet single crystal film grown by an LPE (Liquid Phase Epitaxial) method, in which an external magnetic field is applied in the direction crossing the surface of the single crystal film to cause magnetic saturation, and then, even if the external magnetic field is removed, the Faraday rotation effect at the time of the magnetic saturation is held. It is disclosed that in this single crystal film, when the external magnetic field not lower than the saturation magnetization is applied, even if the external magnetic field is removed, the Faraday rotation angle is kept.
As described above, the bismuth substitutional rare earth iron garnet single crystal film having the hard magnetism is proposed. In this hard magnetic single crystal film, differently from the conventional soft magnetic single crystal film requiring the disposition of the permanent magnet 12, a magnetization direction becomes very important. That is, in the conventional optical isolator 10 shown in FIG. 19, since the magnetization direction of the Faraday rotator 11 made of the soft magnetic single crystal film is determined by the direction of the permanent magnet 12, the front and back surfaces of the Faraday rotator 11 do not especially need to be discriminated from each other. However, in the optical device which does not require the disposition of the permanent magnet 12 and uses the Faraday rotator made of the hard magnetic single crystal film, it is necessary to discriminate the magnetization direction of the Faraday rotator to which the external magnetic field is applied, that is, the front and back surfaces of the Faraday rotator. If the optical device such as the optical isolator is mistakenly assembled with respect to the magnetization direction of the Faraday rotator, the optical device does not function at all.
However, since the front and back surfaces of the Faraday rotator have the same color, discrimination with the naked eye is difficult. In the fabrication of the Faraday rotator, after giving the hard magnetism, a process such as working or washing follows, and it is not easy to continue to discriminate between the front and back in this subsequent process. Thus, JP-A-10-115815 proposes that the hue of an antireflection film formed on the Faraday rotator is made different between the front and back surfaces.
More specifically, after a grown single crystal film is cut and polished, antireflection films having different hues are formed on the front and back surfaces to make a definite form product in which the hues of the front and back are different from each other, and then, the hues of the front and back are used as an index to enable discrimination of the magnetization direction after magnetizing.
However, in the foregoing JP-A-10-115815, when the Faraday rotator is incorporated in the optical device such as the optical isolator, it is necessary to always grasp which hue indicates the front surface or back surface of the single crystal film. Besides, in order to provide coatings having different hues on the front and back surfaces of the single crystal film, the antireflection films must be made to have different structures (kind of medium, or thickness) between the front and back surfaces, and there is a problem in expediency.
Besides, although hard magnetic garnet is not an object, as a method of discriminating the front and back of a Faraday rotator, JP-A-2000-89165 proposes that when magnetic garnet single crystal is cut vertically and horizontally and is worked into a rectangular plate chip, groove working is performed from one surface along one cut line, so that a slit is formed along one side.
Both the proposal of JP-A-10-115815 and the proposal of JP-A-2000-89165 are evaluated in that the front and back surfaces of the Faraday rotator can be discriminated. However, in the proposal of JP-A-10-115815, with respect to the antireflection films provided on the Faraday rotator, although it is originally sufficient if the same is used for the front and back surfaces, to dare to use the antireflection films with the different hues causes such a burden that the design of the antireflection films and the setting of film formation conditions must be made for each of the front and back surfaces. Besides, according to the study of the present inventors, it can be difficult even for a skilled person to recognize the difference according to the hues of the front and back surfaces.
Besides, like the proposal of JP-A-2000-89165, in the Faraday rotator in which the slit is formed along one side by performing the groove working from the one surface along the one cut line, according to the study of the present inventors, it has been confirmed that the characteristics required for the hard magnetic Faraday rotator, especially holding power is lowered.